The present invention relates to a control for avoiding interference between a piston and a valve in an internal-combustion engine having a variable lift mechanism that is capable of changing a lift amount of the valve of the engine and a variable phase mechanism that is capable of changing a phase of the valve.
Some variable valve-driving units for an engine comprise a mechanism that is capable of variably controlling a lift amount of an intake/exhaust valve (hereinafter referred to as a “valve”) of the engine and a mechanism that is capable of variably controlling a phase of the valve. By controlling the lift amount and the phase, an air amount introduced into the engine can be controlled, and hence a desired engine output can be achieved.
When the lift amount and/or the phase are changed, interference (collision) may occur between the valve and a piston of the engine. Several techniques for avoiding such interference have been proposed.
According to one example, a recess is formed at the upper portion of the piston so that the recess can accommodate a head of the valve. According to another example, a range within which the lift amount and the phase can be changed is limited so as to prevent interference between the valve and the piston.
According to a technique disclosed in the Japanese Patent Application Publication No. 2002-332876, it is determined whether or not a difference between an actually measured value and a desired value of the lift amount of the valve is greater than a predetermined value. If it is determined that the difference is greater than the predetermined value, the phase of the valve is retarded by a predetermined amount because there is a possibility that an interference warning line may be exceeded (that is, overshooting may occur).
In the case of providing the recess at the upper portion of the piston, the emission may deteriorate because noncombustible gas is easy to remain in the recess. Such noncombustible gas may increase the combustion time, which may reduce the efficiency of the combustion.
According to the technique of limiting the range for changing the lift amount and the phase, optimizing the overlap between the intake valve and the exhaust valve may be difficult. If such optimization of the overlap is not achieved, residual gas (internal EGR) may increase, which may inhibits the improvement of the fuel efficiency.
Since a desired engine output can be achieved by controlling the lift amount and the phase, a throttle valve can be removed, thereby decreasing the pumping loss. However, if the range for changing the lift amount and the phase is limited, lift amount/phase control alone may not achieve a desired engine output.
The above-described problems may be partially resolved by the above technique disclosed in the Japanese Patent Application Publication No. 2002-332876. However, according to this technique, since a determination of whether the interference avoiding action is started is based on a difference between the actually measured value and a desired value of the lift amount, the avoidance may be delayed and hence the interference may occur between the valve and the piston when the lift amount or the phase sharply overshoots. In order to avoid such interference, it is required to make a range where interference needs to be warned wider. However, under such a wider interference warning range, the optimal lift amount and phase may not be used for operating the engine.
Therefore, there is a need for a control that is capable of avoiding interference between a valve and a piston without delay while operating an engine with an optimal lift amount and an optimal phase.